I feel inclined to point out Miranda's more 007 than GI Joe and most of ME2's action takes place in urban hubs. Moreover, you don't know how protective her 'catsuit' is (it's the future, it's likely tougher than it looks) and whether armor fits her fighting style. The whole 'more armor = good' idea doesn't rest in solid ground.
If catsuits provided as much protection as heavy armor, no one would wear heavy armor. The Alliance would have all of its soldiers in catsuits instead. Its a fairly reasonable assumption that armor designs that look like armor, provide more protection than the catsuits...otherwise the lighter protection would have won out and rendered the other completely obsolete.
The biggest argument I've seen over the years in support of the whole catsuit-in-combat thing, is that kinetic barriers supposedly provide enough protection that armor may not be entirely needed. I think that argument fails for a couple reasons. The first is that by only rolling with kinetic barriers, you're still sacrificing an extra level of protection no matter how powerful those barriers are. The second is that according to the lore the barriers function by using mass effect fields to deflect away fast moving projectiles. The way kinetic barriers function should mean they should provide no protection against the concussive effects of being in close proximity to a grenade, mortar, tank, or artillery shell exploding. Additionally those sorts of explosives also throw out shrapnel from the explosive itself, and can kick up debris from the ground like rocks or pieces of timber....those fragments, often tumbling much slower than bullets (and definitely travelling slower than rounds fired by mass accelerators) can maim or kill. The way kinetic barriers are described may mean they are not very effective at deflecting the shower moving shrapnel. Gameplay, such as pistols greater effectiveness against barriers or hand-to-hand combat bypassing it...heavily implies it. At the very least kinetic barriers certainly won't protect against concussion.
In contrast one of the functions of body armor is to protect against the concussive effects of explosions or the shrapnel they throw or kick up. In fact that is the primary function of modern day body armor like kevlar helmets or flak vests.